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## Summary Improve scrolling performance in the Messages channel list by avoiding unnecessary inventory rebuilds as the header moves. Keep scroll-driven header state in a notifier and retain the inventory while the header animates. Provider and parent updates still rebuild the inventory with current channel data. This complements the lazy-row optimization in #9401. In manual iPhone runs with 500 channels, adding lazy rows on top of header isolation reduced maximum observed Flutter UI work from **65.5 ms to 4.2 ms**. That measurement reflects the added lazy-row change; no standalone performance gain was established for header isolation. This is an independent production-only change. Test fixtures and diagnostic instrumentation are excluded; regression checks were run locally and are not included in this PR. ### Related issue Related: #5911 addresses scrolling behavior inside conversations. This change addresses header-triggered rebuilds in the Messages channel inventory. No matching open PR or issue was identified in the focused duplicate search. ### Testing Exercised Messages on a physical iPhone with 500 channels against an isolated local relay. Header collapse and channel updates worked. This change alone did not remove the large scroll stalls; lazy section rows are a separate change. The device run preceded the refresh onto current main. Local-only regression checks passed 10 tests on this independent branch. They verify stable inventory identity during header movement and visible channel updates. These checks are excluded from the commit. Remaining visible jumps are under investigation; this PR does not claim to resolve them. Generated with Codex Signed-off-by: Michael Fransen <mfransen@squareup.com> Co-authored-by: Karl <9e8471f7b792e46c9c100840959dc71c660786c79c2a3165dbcaf58f52c4266e@buzz.block.builderlab.xyz>